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Bureau of Mines Information Circular/1985 




Earth Grounding Beds— Design 
and Evaluation 

Proceedings: Bureau of Mines Technology Transfer 
Seminars, Pittsburgh, PA, June 25, 1985, 
and Reno, NV, June 27, 1985 



Compiled by Staff, Bureau of Mines 




UNITED STATES DEPARTMENT OF THE INTERIOR 



*W/NES 75TH AV^ 



\.- J trl 






Information Circular 9049 



Earth Grounding Beds— Design 
and Evaluation 



Proceedings: Bureau of Mines Technology Transfer 
Seminars, Pittsburgh, PA, June 25, 1985, 
and Reno, NV, June 27, 1985 



Compiled by Staff, Bureau of Mines 




UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 



i\0' v 



Library of Congress Cataloging in Publication Data: 



Bureau of Mines Technology Transfer Seminars (1985 : 
Pittsburgh, PA, and Reno, NV) 

Earth grounding beds— design and evaluation. 

(Bureau of Mines information circular ; 9049) 

Bibliography: p. 24. 

Includes index. 

Supt. of Docs, no.: I 28.27: 9049. 

1. Mines and mineral resources— Electrical equipment. 2. Elec- 
tric currents— Grounding. 3. Electricity in mining. I. United States. 
Bureau of Mines. II. Title. III. Series: Information circular (United 
States. Bureau of Mines) ; 9049. 

TN295.U4 622s [622\48] 85-600152 










Q 



^ 



PREFACE 



This publication both complements and supplements Bureau of Mines 
Information Circular 8767, "Guide for the Construction of Driven-Rod 
Ground Beds." The soil resistivity and bed resistance measurement pro- 
cedures referenced herein are fully explained in IC 8767. It is recom- 
mended that the reader be familiar with both documents before designing 
a ground bed. Together, they will facilitate the selection of the best 
types of bed (rod, borehole, or composite) for a particular application. 



iii 



CONTENTS 

Page 

Preface i 

Abstract 1 

Introduction 2 

Guide for the construction of borehole ground beds, by H. W. Hill, Jr 3 

Composite ground beds for high-resistivity soils, by M. R. Yenchek 8 

Procedure for the direct measurment of touch potentials, by W. L. Cooley, 

H. W. Hill, Jr., and M. L. McBerry 12 

References 24 





UNIT OF MEASURE ABBREVIATIONS 


USED IN THIS 


REPORT 


A 


ampere 


m 


meter 


ft 


foot 


mA 


milliampere 


ft* 


square foot 


ohm-f t 


ohm-foot 


Hz 


hertz 


pet 


percent 


in 


inch 


V 


volt 


kV 


kilovolt 


V/A 


volt per ampere 



EARTH GROUNDING BEDS-DESIGN AND EVALUATION 

Proceedings: Bureau of Mines Technology Transfer Seminars 
Pittsburgh, PA, June 25, 1985, and Reno, NV, June 27, 1985 

Compiled by Staff, Bureau of Mines 



ABSTRACT 

Recent Bureau of Mines research in earth grounding safety is document- 
ed in three separate papers. First, guidelines for the design of a 5- 
ohm borehole ground are presented. Next, an inexpensive practical means 
to construct a low-resistance bed in high-resistivity soil is described. 
Finally, a procedure that employs a commercial resistivity meter to mea- 
sure shock potentials around grounded objects is explained step-by-step. 
These papers complement the material presented at an earlier technology 
transfer seminar and published as IC 8767. 



INTRODUCTION 



It has been established that the re- 
sistance of mine ground beds should be 
designed as low as practical, with 5 ohms 
as a generally acceptable value (1-2) . 1 
Step-by-step procedures for designing a 
5-ohm, driven-rod bed are documented in a 
Bureau of Mines Information Circular (IC 
8767) (_3 ) . However, in certain cases, 
the driven-rod bed may not be the best 
choice for maximum protection or cost 
effectiveness . 

For instance if land area is limited or 
if step and touch potentials are a prime 
concern around the surface substation, an 
existing borehole may be more suitable as 
the earth electrode than a driven rod. 
Further, if soil resistivity exceeds 500 
ohm-ft, a rod bed may be impractical from 
an economic standpoint. In such a case a 
composite bed utilizing low-resistivity 
fill may be the best option. 

Consequently, one of the purposes of 
this publication is to guide the bed 

'Underlined numbers in parentheses re- 
fer to items in the lists of references 
at the end of the final paper in this 
report. 



designer in selecting the most appro- 
priate bed type for each particular 
situation, given the options of driven 
rods, an existing borehole, or composite 
materials. The papers by Hill and Yen- 
chek, reporting on borehole and composite 
ground beds, respectively, act as a sup- 
plement to IC 8767, which covers the 
driven-rod option. 

Once a ground bed has been constructed, 
its resistance has traditionally been the 
gauge for the safety of the entire 
grounding system. However, recent Bureau 
research has shown that mine grounding 
systems may pose dangers to personnel 
under certain conditions, despite a low 
bed resistance. These hazards may arise 
during phase-to-earth faults and take the 
form of dangerously high step and touch 
potentials around grounded equipment at 
locations remote from the substation. 

In the final paper from this seminar, 
Cooley, Hill, and McBerry explain a new 
method for evaluating these potential 
hazards without interfering with mine op- 
erations. As such, this paper serves as 
a complement to IC 8767 and the first two 
papers in this publication. 






GUIDE FOR THE CONSTRUCTION OF BOREHOLE GROUND BEDS 
By H. W. Hill, Jr. 2 



ABSTRACT 

Using the resistivity measurement tech- 
niques of IC 8767 (_3 ) , this guide is in- 
tended to help the user decide between 
a ground bed constructed from rods and 
one consisting of a borehole. Cost and 
safety issues for both types of beds are 
discussed. For the engineer who decides 
to build a borehole ground, design tables 
are included for a 5-ohm ground bed in 
earth of various resistivities. Design 
verification procedures are given that 
are tailored for the borehole ground bed. 

INTRODUCTION 

The construction of any ground bed 
is not an exact procedure. Two formi- 
dable obstacles keep the design meth- 
ods from being exact: the complex ways 
that current and voltage vary within the 
earth and the practical difficulties of 
determining the earth resistivity in 
sufficient detail to determine these 
variations. To accurately predict the 
resistance of a ground-bed design, the 
electrical resistivity of every cubic 
inch of earth at the proposed site should 
be known. These millions of values (that 
can only be obtained from surface mea- 
surements) must be then used in a mam- 
moth computer program to precisely pre- 
dict the resistance of the completed bed. 

These limitations have more significant 
implications for borehole ground beds 
than for rod beds, because the resistiv- 
ity is easily measured only at the 
earth's surface. Consequently, more in- 
formation is available about the re- 
sistivity near the surface than at great 
depths (although interpretation of sur- 
face measurements yields some estimates 
of subsurface resistivity). Borehole 
ground beds extend farther beneath the 
surface and thus depend more on the 

^Associate Professor, Department of 
Electrical and Computer Engineering, Ohio 
University, Athens, OH. 



subsurface resistivity than rod beds. 
The end result is that the design proce- 
dures and_ tables presented here for bore- 
hole beds are NOT as dependable as those 
for driven-rod beds in IC 8767, so that a 
conservative design philosophy is even 
more important. 

CHOOSING BETWEEN BOREHOLES 
AND DRIVEN RODS 

Either boreholes or driven rods can 
serve equally well as the basis of a sat- 
isfactory ground bed in most installa- 
tions. However, there are special situa- 
tions for which one or the other is a far 
superior choice. This section addresses 
some of those situations. 

Installations With Existing Boreholes 

A mine with existing borehole(s) should 
consider the impact that these bore- 
hole(s) have on the mine safety grounding 
system, even if they are not presently 
connected to the grounding system, and 
whether or not the existing grounding 
system is adequate. This is particularly 
true of underground mines, where (for in- 
stance) boreholes may be used to supply 
power to rail haulage at various points, 
or where metal water lines make their way 
out of the mine at different locations. 
The fact is that these pipes and borehole 
casings are capable of transferring volt- 
ages from the surface to underground, re- 
gardless of their reason for existence. 

This subject is discussed in some de- 
tail in Bureau IC 8835, "Guide to Substa- 
tion Grounding and Bonding for Mine Power 
Systems" (4^). The basic consideration is 
that adequate protection from electrical 
shock cannot be provided to personnel at 
both ends of a borehole casing. Borehole 
casings within a substation should be 
connected to the substation ground and 
avoided by personnel underground. Bore- 
holes more than 50 ft from the substation 
can be connected to the safety ground 
system. If boreholes are not connected 



to the grounding system in any way, per- 
sonnel should avoid contact with them. 

New Installations and Installations 
With Excessive Bed Resistance 

When a new ground bed must be built, 
either for a new installation with no ex- 
isting bed or in an older installation 
with a seriously deficient ground bed, 
concerns for safety should have the high- 
est priority in picking the type of bed 
to be built. The most important safety 
advantages and disadvantages of borehole 
ground beds relative to driven-rod beds 
are discussed below. 

Advantages 

1. Lower standard touch potentials on 
the surfaces : The potential around a 
borehole electrode (carrying ground-fault 
current) decreases logarithmically with 
the distance from the borehole, whereas 
the potential around a rod bed decreases 
with the reciprocal of distance. The 
latter variation is more dramatic, caus- 
ing higher differences in voltage. Fig- 
ure 1-1 illustrates this difference be- 
tween bed types. 

2. Capable of achieving lower resist- 
ance : For a given fault current, the 
lower the ground-bed resistance, the low- 
er the fault voltage will be. Therefore, 
a borehole is safer if it yields a lower 
resistance. Because the resistance of a 
borehole depends on subsurface resistiv- 
ity that may be significantly lower than 
surface resistivity, a borehole ground 
bed may have a lower resistance than a 
rod bed. 

3. Less seasonal variation in resist- 
ance : Soil has a substantially higher 
resistivity when it freezes. Boreholes 
typically extend far beneath frost pene- 
tration depths, so the resistance does 
not change as radically in winter as does 
the resistance of rod beds. 

Disadvantages 

1. Higher step and touch potentials 
underground : When fault current flows 
from a ground bed into the earth, the 
ground near the bed becomes elevated in 



potential almost to the same extent as 
the bed itself. Because the borehole ex- 
tends deeper into the ground than do 
other bed types , voltages are transferred 
deeper underground also. 

2. Less lightning protection than rod 
beds : Although lightning may strike a 
borehole casing, particularly in high- 
resistivity soil, lightning currents will 
typically not be conducted down a long 
borehole casing if they first strike 
another conductor connected to the bore- 
hole. This can be interpreted to mean 
that the ground bed has a higher resist- 
ance to lightning than a rod bed, even 
though the rod bed may measure the same 
or higher resistance in a test. 

3. More difficult to design and mea- 
sure adequacy : Ground-bed design is no 
more accurate than the resistivity data 
on which it is based. Borehole design 
requires subsurface resistivity data that 
typically are not well known. 

4. Joints in casings can suddenly in- 
crease resistance : Borehole casings used 
as grounds can be treacherous , because 
joints between sections may not provide a 
good electrical contact. Often, newly 
constructed boreholes will provide elec- 
trical continuity from end to end, but 
this continuity will not be maintained as 
the borehole ages. Frequent measurement 
is recommended for this type of ground 
bed. 

Cost Advantages and Disadvantages 
of Borehole Grounds 

If there is no clear choice on the ba- 
sis of safety for picking boreholes or 



100 




20 40 60 90 

BOREHOLE LENGTH, ft 
FIGURE 1-1. - Bed voltage versus distance. 



100 



driven rods, the cost issue should be ex- 
amined. The advantages and disadvantages 
of boreholes in terms of cost are sum- 
marized below. 

Advantages 

1 . Less real estate committed to bore- 
hole ground bed: Obviously, it takes 
less surface area for one borehole casing 
than it does for a bed of driven rods, 
since each rod must be separated by prac- 
tical distances from one another. How- 
ever, this advantage is somewhat offset 
by the fact that the voltage is higher 
between a borehole ground and any other 
ground bed than between driven rods and 
any other ground bed. Therefore, for 
safety reasons a borehole ground must be 
placed farther from the substation than a 
bed consisting of driven rods. Figure 
1-1 quantifies this necessary difference 
in required spacing; for example, with a 
maximum voltage of 50 pet the nearest rod 
may be only a few feet from the substa- 
tion, whereas a borehole would have to be 
about 20 ft away. (The coupling between 
beds is identical to the voltage shown on 
the vertical axis.) 

2. Less material if subsurface resis- 
tivity is low : The amount of material 
necessary to construct a ground bed of a 
given resistance is directly proportional 
to the resistivity of the earth in con- 
tact with the bed. If the subsurface re- 
sistivity is substantially lower than the 
surface resistivity, a borehole ground 
extending into this lower resistivity re- 
gion may require less material than a rod 
bed on the surface. 

Disadvantages 

1. The borehole construction technique 
is more expensive per electrode than the 
driven-rod techniques. 

2. Elaborate methods are necessary to 
assure earth contact. 

Both disadvantages are consequences of 
the differences in construction between 
boreholes and rod beds. Because the rods 
are driven into the earth, no hole has to 
be made for them in advance. Also, be- 
cause the rod is forced into the earth, 



the contact between the earth and the rod 
tends to be quite good. 

If, after reviewing the safety and cost 
factors listed above, the borehole re- 
mains a viable choice , then a borehole 
ground can be designed using the method 
outlined in the next section. After com- 
paring this borehole design with a rod 
bed design (using IC 8767), a final deci- 
sion can be made. 

DESIGN OF A BOREHOLE GROUND BED 

Design of any ground bed begins with 
measurements of earth resistivity. It is 
recommended that the borehole ground bed 
design begin with the same resistivity 
measurements that are detailed on pages 
10-13 and illustrated in figures A-l 
through A-4 of IC 8767. Particular at- 
tention should be paid to the results of 
measurement B and measurement D. The 
lower these numbers are relative to mea- 
surements A and C, the more suitable-* the 
proposed site is for a borehole ground. 
Of course, a borehole ground bed can be 
constructed even if measurements A and C 
yield lower resistivities than the other 
tests. 

The preliminary design is carried out 
by averaging the resistivities obtained 
in measurement B and measurement D. This 
average value is used with table 1-1 to 
obtain the tentative dimensions of a 
borehole ground. 

If the average resistivity falls be- 
tween two values in the table, the higher 
value should be used. For example, if 
measurement B yielded 550 ohm-ft and mea- 
surement D yielded 470 ohm-ft, the aver- 
age is 510 ohm-ft, so the line in the ta- 
ble corresponding to 700 ohm-ft should be 
used. Any one of the combinations of 
borehole length and diameter in table 1-1 
should be satisfactory, except those 
marked with asterisks, which are shown 
for illustrative purposes only. Choice 
among the alternatives here can be made 
on the basis of available materials and/ 
or existing boreholes. 

■^More suitable here means more effi- 
cient use of material. 



TABLE 1-1. - Minimum borehole lengths for a 5-ohm bed, feet 



Minimum 




Maximum 


earth 


resistivity, 


ohm- ft 




diameter, in 


100 


200 


300 


500 


700 


1,000 


2,000 


3,000 


1 


22 


49 


79 


140 


205 


305 


660 


1,032 


2 


20 


44 


71 


128 


187 


281 


611 


959 


4 


17 


29 


63 


115 


170 


256 


561 


885 


6 


15 


36 


59 


108 


159 


241 


532 


842 


8 


14 


34 


55 


102 


152 


230 


511 


811 


12 


12 


31 


51 


94 


141 


215 


481 


767 


16 


11 


28 


47 


89 


134 


204 


460 


735 


20 


10 


26 


45 


85 


128 


196 


444 


711 


24 


'9 


25 


42 


81 


123 


189 


430 


690 


30 


'8 


23 


40 


77 


116 


180 


413 


666 



'Questionable values because they are beyond the range of 
validity of underlying assumptions. 



The second step in the design is to 
carry out additional resistivity measure- 
ments, with electrode spacings as close 
as practical to the length of the chosen 
borehole in table 1-1. If the borehole 
chosen has a length of 25 ft or less, 
this step can be omitted because measure- 
ments B and D were done with 18-ft spac- 
ings. If the electrode spacings cannot 
be increased to the specified borehole 
length, then the maximum electrode spac- 
ings should be used. The measurement 
taken along the same baseline as measure- 
ments A and B will be referred to as mea- 
surement E; the new measurement made per- 
pendicular to the baseline (that is, the 
same line as measurements C and D) will 
be referred to as measurement F. 

Making measurements E and F with short- 
er spacings than the borehole length will 
introduce more error into a process that 
is already uncertain; the final design 
should be made more conservative (longer 
borehole, larger diameter) if the mea- 
surements must be made with short spac- 
ings. The larger the discrepancy between 
measurements E and F and measurements B 
and D, the more uncertainty there is. 

If the results of measurement E and F 
are within 20 pet of each other, then the 
average of these results should be used 
to pick a new design from table 1-1. 
Otherwise, the larger of the two results 
should be used. Hopefully, this design 
will be the same as the initial one 
chosen above. However, if the results 
of these latter measurements are very 



different from measurements B and D, the 
same design cannot be used. 

If the borehole length in the new de- 
sign differs by more than 20 pet from the 
length in the first design, the possibil- 
ity of making more resistivity measure- 
ments should be considered. This is par- 
ticularly important if the new design 
calls for a longer length than the ini- 
tial design, and the maximum possible 
electrode spacing was NOT used in mea- 
surements E and F. The results of the 
repeated measurements should be used in 
picking a third design (that, hopefully, 
will be the same as the second design). 

DESIGNING A BOREHOLE GROUND 
FOR OTHER THAN 5 OHMS 

Table 1-1 was computed for 5-ohm ground 
beds, because the majority of safety 
ground beds are designed for this value. 
However, the table can also be used for 
other design values. The procedure is 
very similar to the one described in IC 
8767. Each measured resistivity is di- 
vided by the fraction of 5 ohms repre- 
sented by the desired ground-bed resist- 
ance. If measurements B and D yield an 
average of 400 ohm-ft, and a 4-ohm ground 
bed is desired, the 400 ohm-ft must be 
divided by 4/5. Table 1-1 would then be 
used with the "fictitious" value of 500 
ohm-ft. (The same procedure would be 
followed with the results of measurements 
E and F.) 






MEASUREMENT OF BOREHOLE RESISTANCE 

Once the ground bed is designed and 
built, its resistance must be measured to 
ensure that the desired resistance value 
has been achieved. Ground-bed resistance 
measurement is described in detail in IC 
8767, pages 4-6. It is illustrated in 
figure 4 of that publication, with sample 
results shown in figure 5. The primary 
concern in measuring the resistance of a 
borehole ground is to locate the aux- 
iliary current electrode at a sufficient 
distance (D) from the borehole. Prefer- 
ably, the current electrode should be 
placed more than five borehole lengths 
from the borehole. In this case, the 
procedure described in IC 8767 can be 
used without modification to find the re- 
sistance of the borehole ground. 

Because borehole lengths are typically 
on the order of several hundred feet, 
frequently the current electrode cannot 
be placed 5 times this distance from the 
ground bed. Equipment limitations and 
unfavorable terrain are the most common 
constraints on electrode placement. The 
f all-of-potential measurement procedure 
must be modified if the current electrode 
is to be placed closer than specified 
above. 



The current electrode should be posi- 
tioned at a distance, D, that corre- 
sponds to one of the positions in table 
1-2; that is, it should be located either 
at 1/2, 2/3, 1 or 2 borehole lengths 
from the borehole. For any of these 
positions, the f all-of-potential mea- 
surement is carried out as specified in 
IC 8767, but the potential-electrode po- 
sition used for the resistance determi- 
nation is not 0.618 times the current- 
electrode spacing (as given in IC 8767) 
but rather the fraction indicated in ta- 
ble 1-2 times the current-electrode spac- 
ing. The smaller the current-electrode 
spacing used, the more questionable is 
the resistance measurement. 

TABLE 1-2. - Potential-electrode 
location for borehole resistance 
measurement 

Potential-electrode 
position 2 
Current-electrode 
position: ' 

2 0.602 

1 .570 

2/3 .538 

1/2 .509 

'Multiple of borehole length. 
2 Fraction of distance (D). 



CONCLUSION 



This paper has expanded but not re- 
placed the content of IC 8767 to provide 
the user with additional choices for de- 
signing and building a ground bed. The 
discussion presented here of advantages 
and disadvantages of borehole grounds 



should foster an intelligent choice of 
grounding technique. The material pre- 
sented on ground-bed measurement will en- 
able the user to verify the adequacy of a 
chosen design. 



COMPOSITE GROUND BEDS FOR HIGH-RESISTIVITY SOILS 
By M. R. Yenchek 4 



ABSTRACT 

An inexpensive, practical means to con- 
struct a low-resistance ground bed where 
soil resistivity exceeds 500 ohm-ft is 
described. The technique involves using 
a large quantity of semiconducting fill 
material in contact with a relatively 
small, metal-grounding electrode. This 
composite material bed is shown to be 
superior to conventional rod beds from 
both safety and economic standpoints. 
The design of a 5-ohm, circular-ring 
composite bed is explained step-by-step, 
beginning with soil resistivity measure- 
ments and ending with a check on bed re- 
sistance. The resistivities of fill ma- 
terials commonly found near mine sites 
are listed. 

INTRODUCTION 

The earth connections of power distri- 
bution systems, typically ground beds, 
protect personnel and equipment from many 
operational hazards. A properly designed 
bed exhibits low resistance to limit the 
potentials of the metallic frames con- 
nected to it and to facilitate activation 
of ground-fault protective devices. In 
addition, it minimizes voltage gradients 
during lightning strikes and phase-to- 
earth faults. 

Many mining sites are located in dry, 
rocky terrain where the soil exhibits 
high resistivity; values greater than 
3,000 ohm-ft have been measured (5). 
Since bed resistance is directly propor- 
tional to soil resistivity, construction 
of a low-resistance ground bed by conven- 
tional methods may be difficult in these 
areas. For example, to build a 5-ohm, 
driven-rod bed in 3,000 ohm-ft soil re- 
quires one hundred 10-ft rods distributed 
over 5-1/2 acres (3). A bed of this 

^Electrical engineer, Pittsburgh Re- 
search Center, Bureau of Mines, Pitts- 
burgh, PA. 



magnitude is not only expensive but very 
impractical. Even if such a bed were to 
be constructed, dangerous potentials from 
fast-rise-time wavefronts, i.e., light- 
ning, would be likely when the bed would 
conduct current. 

What is needed is an inexpensive, prac- 
tical means to construct a low-resistance 
ground bed in high-resistivity soils. An 
alternative to using only metal as the 
grounding electrode would be to use a 
large quantity of an inexpensive, low- 
resistivity, semiconducting material in 
contact with a relatively small metal 
electrode. This fill and metal-electrode 
combination comprises a composite ground 
bed. This paper shows that such a design 
can be superior to conventional rod beds 
in high-resistivity soils. 

ADVANTAGES OF THE COMPOSITE GROUND BED 

A composite ground bed can take many 
shapes. If there is a natural depression 
at the proposed site, fill can be dumped 
into it to cover the metallic electrodes; 
on level ground the fill can be mounded. 
The grounding electrodes can be vertical 
rods or horizontal conductors. 

One practical composite design in the 
form of a circular ring is shown in 
figure 2-1. Here a copper conductor 
(typically 1/0 to 4/0 AWG) is buried in 
contact with a low-resistivity fill ma- 
terial. The advantages of a composite 
bed become apparent if we analyze the 
circular ring configuration in 1,000- 
ohm-ft soil. 

We can achieve a 5-ohm bed resistance 
by constructing a circular composite bed 
using low-resistivity material from a 
sanitary landfill. The radius of the 
fill material surrounding the metal con- 
ductor need only be about 2.5 ft and the 
metallic ring radius about 60 ft. In 
contrast, the radius of a 5-ohm wire ring 
directly buried in 1,000-ohm-ft soil must 
be over 100 ft (6). 




TOP VIEW 



Conductor 




Earth \ \(Not to scale: 
SIDE VIEW 
FIGURE 2-1. - Circular ring composite ground bed. 



the slope, the greater the shock hazard 
to personnel near the bed. Notice that 
the profile for the nonfill bed has a 
steeper slope particularly near the metal 
ring. 

Thus, the use of fill materials reduces 
the magnitude of voltage gradients near 
the bed. For the examples in 1,000- 
ohm-ft soil, it can be shown that gradi- 
ents near the composite bed are one-half 
those" near the nonfill bed (5). 

The costs of installing a ground bed 
must include the costs of excavation and 
materials. Excavation costs are incurred 
for both the composite and nonfill de- 
signs. The composite bed is economical- 
ly feasible if low-resistivity fill is 
available near the bed site. Generally, 
if fill costs including transportation 
can be limited to less than 6 times the 
costs of the ring excavation, a composite 
bed is a good choice (6). 




107 109 III 113 115 117 
DISTANCE FROM CENTER OF RING, ft 

FIGURE 2-2. - Voltage profiles for composite 
and nonfill ring designs. 

The effect of the composite fill mate- 
rial is more striking when the voltage 
profile of the surrounding earth is exam- 
ined. This effect is shown in figure 2-2 
for both the composite and nonfill ring 
designs (6^). The profile slope is an in- 
dication of the severity of the poten- 
tials associated with the bed during cur- 
rent flow through the earth — the steeper 



COMPARISON WITH CONVENTIONAL ROD BEDS 

To build a 5-ohm bed using metallic 
rods in 1,000-ohm-ft soil, a 9 by 9 array 
of 8-ft rods (81 rods) spaced as in fig- 
ure 2-3 is required (6). The greatest 
voltage gradients under fault conditions 
would occur around the bed perimeter 
[1.78 V/A of fault current (6)]. As the 
vertical rods must be interconnected hor- 
izontally, excavation costs are signifi- 
cant and, generally, far exceed the cost 
of the rods. The horizontal interconnec- 
tions reduce bed resistance by only 15 
pet, so the overall bed size would not 
change appreciably (7). 

This 5-ohm rod design for 1,000-ohm-ft 
soil is compared with the circular-ring 
composite bed design in table 2-1 (5). 
Note that the composite bed requires more 
land than the rod bed. This drawback is 
offset by the fact that a composite cir- 
cular-ring ground may be constructed at a 
much lower cost if low-resistivity fill 
is readily available. More importantly, 
the maximum potentials associated with a 
rod bed are nearly three times those of 
the composite design, an important con- 
sideration in substation design. So, for 
high-resistivity soils the composite cir- 
cular ring is a realistic alternative. 



10 



xxxxxxxxx 
xxxxxxxxx 
xxxxxxxxx 
xxxxxxxxx 
xxxxxxxxx 
xxxxxxxxx 
xxxxxxxxx 
xxxxxxxxx 
xxxxxxxxx 



P- 1,000 ohm-ft 
Rod length ■ 8 ft 
Rod radius 0.0208 ft 
81 rods 
12.7- ft rod spacing 

FIGURE 2-3. - Driven-rod ground bed. 



TABLE 2-1. - Comparison of conventional 
rod and composite ring beds 





Ground 


Maximum 




area, ft 2 


gradient, V/A 


Conventional 








10,404 


1.78 


Composite cir- 






cular ring. . . . 


14,400 


.77 



RESISTIVITIES OF COMMON 
FILL MATERIALS 

The fill material used in a composite 
ground bed must be inexpensive and should 
have a resistivity less than 200 ohm-ft. 
Fill materials commonly found near mine 
sites are given in table 2-2 (6^). For 
a given bed resistance, the lower the 
fill resistivity,- the smaller the bed 
dimensions. 



TABLE 2-2. - Resistivities of common 
fill materials 

Resistivity, ohm-ft 





100 


Sanitary landfill.... 


10-45 


Steel-mill slag pile. 


150 




15-160 



DESIGN OF A COMPOSITE GROUND BED 

The resistance of the composite circu- 
lar-ring ground bed (6) is given by — 



R oo = 



2tt 2 r 



8r 



In 



8r 

- In — 
a f J 



2tt 



8r 
In — 



(1) 



where 



R » is the bed resistance with 
respect to infinite earth, 

p is the earth resistivity, 
p is the fill resistivity, 



l f 



is the fill radius, 



and 



a is the metallic conductor 
radius, 

r is the ring radius. 



This expression was analyzed for vari- 
ous earth and fill resistivities assuming 
a 2/0 conductor and R °° set at 5 ohms. 
The results are graphed in figure 2-4 for 
reference in the design process. 

Generally the composite bed design is a 
good choice if — 

• soil resistivity exceeds 500 ohm-ft, 

• sufficient land area is available, 

and 

• low-resistivity fill is nearby. 

The design of any ground bed begins 
with soil resistivity measurements. The 
procedure detailed in IC 8767 should be 
referenced for the discussion below. 



11 





"O 20 JO 40 50 

A, ^eorth=500 ohm-fl 



40 50 60 70 80 90 

B, ^eorth= 1,000 ohm-ft 





BO 90 100 IK> 

C, ^eorth = |,500 ohm-ft 



O0 120 140 160 ISO 200 

D, dearth = 2,000 ohm-ft 




"20 -40 *C «C 200 220 240 60 180 200 220 240 260 

£,^ecrth = 2,500 c*im-ft ^eorth,- 3,000 ohm-ft 

RING RADIUS, ft 
FIGURE 2-4. - Radii required for earth resistivity. 

If all four resistivity readings are 
within 20 pet of each other, figure 2-4 
(6^) may be used directly without restric- 
tions. Simply look up the nearest earth 
and fill resistivities and determine the 
appropriate ring and fill radii. 

If the baseline resistivity measure- 
ments A and B (3) are close and the mea- 
surements perpendicular to the baseline 
(C and D) are close, but differ from A 
and B, then the dimensions of the compos- 
ite ring should be modified to an ellipse 
shape with the axes proportional to the 



ratio (p A + Pb)/(Pc + Pd^* For example, 
if the average resistivity along the base 
line is 1.5 times the average perpendicu- 
lar to the baseline, the radius of the 
ring should be increased by 50 pet along 
the baseline. 

If the resistivity readings at 6-ft 
spacings (A and C) (_3) are close to each 
other, but differ from the 18-ft readings 
(B and D), the average of readings A and 
C should be used in figure 2-4. 

If no more than two of the resistivity 
measurements are close (less than 20 pet 
apart), then the baseline should be moved 
45° clockwise or counterclockwise and all 
four measurements repeated. If these 
numbers differ by more than 20 pet, then 
the highest of the four resistivity mea- 
surements, not the average, should be 
used when referencing figure 2-4. 

MEASUREMENT OF THE COMPOSITE 
BED RESISTANCE 

Once the composite bed has been de- 
signed and built, its resistance should 
be measured using the f all-of-potential 
procedure described in detail in IC 8767 
and other grounding handbooks O, 8). 
The current electrode should always be 
located at least a distance of 10 times 
the ring radius from the center of the 
ring. The bed resistance should be the 
reading obtained when the potential elec- 
trode is about 60 pet of the distance to 
the current electrode (9) . 

CONCLUSION 

This paper has shown how to construct a 
practical, low-resistance ground bed in 
high-resistivity soils. This composite 
design utilizes a large quantity of low- 
resistivity fill in contact with a metal 
electrode. It is presented as an alter- 
native to conventional driven-rod beds 
and is particularly advantageous if suit- 
able fill material is readily available. 



12 



PROCEDURE FOR THE DIRECT MEASUREMENT OF TOUCH POTENTIALS 

By W. L. Cooley, 5 H. W. Hill, Jr., 6 
and M. L. McBerry^ 



ABSTRACT 

The procedure outlines a method by 
which the personnel safety provided by a 
mine grounding system can be estimated in 
a direct way. Actual touch potentials, 
resulting from a small-scale simulated 
fault, are measured. Because it does not 
require that the ground bed be discon- 
nected, the procedure avoids the need to 
interrupt mine production while ground 
system safety is determined, and it pro- 
vides an assessment of the effectiveness 
of the entire ground system, not just the 
isolated ground bed itself. 

The measurements needed are relatively 
simple, requiring the use of a four- 
electrode earth resistance meter, which 
is insensitive to any stray ground cur- 
rent that may be flowing in the area. 
Hazardous touch potentials could occur at 
thousands of points throughout the mine 
property. The bulk of the detail given 
in the procedure provides a straightfor- 
ward method of identifying those few ar- 
eas on the mine property where the most 
hazardous touch potentials are likely to 
occur, and gives step-by-step instruc- 
tions for the verification of these 
points and the estimations of the poten- 
tials that could occur there under fault 
conditions. It guides the user through 
the interpretations of 
suggests ways in which 
reduced. 

INTRODUCTION 



the results and 
hazards can be 



Personnel making electrical measure- 
ments on power systems are always subject 
to some risk of electrical shock. Any 

-^Professor, Electrical Engineering, 

West Virginia University, Morgantown, WV. 

Associate Professor, Department of 

Electrical and Computer Engineering, Ohio 

University, Athens, OH. 

7 

Graduate student, Electrical Engineer- 
ing Department, West Virginia University, 
Morgantown, WV. 



metal object or wire connected to a power 
system should be assumed to be lethal un- 
til it has been tested for voltage. The 
grounding system is not an exception to 
this rule. People have been shocked by 
ground beds. 

Neither the ground-bed resistance meth- 
od nor the technique described here tests 
the ability of the grounding conductors 
to carry the high currents of a phase-to- 
ground fault. Additional physical in- 
spections or high-current tests should be 
performed to verify that the grounding 
system can carry these currents. The 
magnitude of possible fault currents can 
be found from the tests described in the 
last section of this procedure. 



CAUTION 

These methods verify the conti- 
nuity but not the ampacity of a 
grounding system. 

Ground beds may be shock hazards; 
check for voltage between the 
ground bed and a stake driven in 
the earth 3 ft from the bed be- 
fore proceeding with any other 
measurements. 



The degree of safety of a grounding 
system is usually assessed by a measure- 
ment of its ground-bed resistance. An 
alternative technique of directly measur- 
ing shock potentials is presented here. 
Each technique has its merits. The fol- 
lowing guidelines should help determine 
which is more suitable for a particular 
mining application. 

Touch-potential measurement is recom- 
mended when — 

1. It is physically impossible to dis- 
connect the ground bed. 

2. The power system cannot be shut 
down without severe economic penalties. 



13 



3. Ground faults could reasonable be 
expected to occur near the ground bed or 
near equipment grounded by the bed. 

A. The "footprint" of grounded equip- 
ment is comparable to the size of the 
ground bed. ("Footprint" here is defined 
as the outline of the part of the machine 
in contact with the earth.) 

5. The grounding system is not well 
documented and mostly hidden from view. 

Ground-bed resistance measurement is 
recommended when-- 

1. The ground bed is relatively small 
and conveniently disconnected. 

2. The quantity or type of equip- 
ment to be grounded to the bed changes 
frequently. 

3. Equipment connected to the bed 
changes location significantly between 
measurements. 

4. Ground faults are very unlikely to 
occur near the ground bed or grounded 
equipment. 

If a particular power system is better 
characterized by the first list above 
than by the latter one, the procedure ex- 
plained here should be applicable. 

INSTRUMENT SELECTION 

It is recommended that a commercial 
meter built for making ground-bed resist- 
ance measurements be used for the touch 
potential measurement. Any four-terminal 
meter built for this purpose will be 
suitable provided that (1) it uses a mea- 
surement frequency less than 500 Hz; (2) 
it produces an open-circuit voltage (be- 
tween the current terminals) of less than 
100 V; (3) it works reliably with up to 
10 A of stray ac or dc current. The 
first condition assures that the measure- 
ment will be relevant to the power- 
frequency operation of the power system; 
the second makes sure that the instrument 
itself will not be a source of electrical 
shocks; and the last condition insures 
good measurements in the electrically 
hostile mining environment. 

OVERVIEW OF METHOD 

Figure 3-1 shows the basic schematic of 
the method. Essentially, a low-level 



fault (typically 10 to 20 mA) is staged 
using the internal current source of the 
test instrument. Touch potential due to 
this fault is measured by the voltage de- 
tector of the same unit. The instrument 
meter displays touch potential in volts 
per ampere of fault current. This number 
must be multiplied by the largest antici- 
pated ground-fault current to obtain the 
worst-case touch potential. For example, 
if the instrument indicateed 0.100 on a 
system where the maximum ground-fault 
current was expected to be 250 A, the 
maximum touch potential would be 25 V. 

The instrument connections are much 
like those of the f all-of-potential mea- 
surement; personnel familiar with this 
technique should have little difficulty 
with measuring touch potentials. One 
current connection and one voltage con- 
nection are made to the ground bed (or 
grounded equipment) under test, and the 
second current and voltage connections 
are made to test electrodes away from the 
bed (or grounded equipment) . 

Differences between the f all-of-poten- 
tial method and the touch potential meth- 
od lie in the locations of these last 
two electrodes. In the f all-of-potential 
method, the current electrode is located 
as far as possible from the ground bed, 
and the potential electrode is placed at 
various locations on a line between the 
ground bed with the current electrode. 
In the touch-potential measurement, the 
potential electrode is placed 3 ft from 
the ground bed, and the current electrode 
is located where a ground fault may 
occur. 

GENERAL GUIDELINES FOR 
ELECTRODE PLACEMENT 

As explained above, one current connec- 
tion is always made to the ground bed. 
This connection is either made directly, 
if the test is performed near the bed, or 
indirectly, if the measurement is done 
at a piece of grounded equipment. The 
indirect connection is accomplished by 
attaching the test lead to the metal 
frame of the grounded equipment; this es- 
tablishes electrical continuity to the 
ground bed through the ground wire. 

Only voltage connection is also made to 
the ground bed, indirectly or directly 



14 



Overhead lines 
(bare conductor) 




FIGURE 3-1. - Measuring touch potential at a machine with a circular footprint. 



(as above). It is important that this be 
a separate wire from the meter to the bed 
or to the grounded equipment, not a jump- 
er between the two meter terminals. A 
separate clamp should be provided for 
each connection. 

The second current connection should be 
made to a test stake driven into the 
ground, at the closest point to the 
ground bed or any grounded equipment 
where a bare phase conductor could come 
into contact with the earth. If overhead 
lines pass close to remote grounded 
equipment, as well as close to the sub- 
station where the ground bed is built, 
the procedure should be performed in both 
places, even if the distance from the 
remote-grounded equipment to the overhead 
line is larger than the distance from the 
ground bed to the nearest phase-conductor 
grounding point. ^ 

The second potential connection is al- 
so made to a test stake or electrode. 
Placement of this test electrode is 
the most difficult part of the measure- 
ment procedure. It must be placed 3 ft 
from grounded equipment at the point 
which will produce the maximum instrument 



reading (the maximum voltage for a given 
fault current). This location is not al- 
ways obvious, but time spent checking 
different locations the first time the 
measurement procedure is conducted at a 
given mine site should not have to be re- 
peated during subsequent tests. 

The following subsections list some 
general guidelines depending on the type 
of footprint. Another section of this 
paper presents some specific suggestions 
for different mine types. However, these 
guidelines and suggestions are not a sub- 
stitute for experience. Any incidence at 
a mining property of metal objects being 
"live" should be investigated using this 
procedure, by placing the potential con- 
nection at that site. 

°In some instances, the overhead line 
passing by remote grounded equipment may 
be a high-voltage transmission or sub- 
transmission line. A ground fault on one 
of these high-voltage lines may produce 
shock hazards that cannot be eliminated 
even by well-engineered ground beds. 
This procedure will nonetheless help 
quantify these hazards. 



15 



Machine With a Circular Footprint 

When the touch potential is to be mea- 
sured near a ground bed or a grounded 
machine that has a circular footprint, 
the potential electrode should be placed 
as close as possible to the current elec- 
trode (while maintaining the 3-ft spacing 
from the edge of the equipment). The 
most common example of this would be 
a dragline. Note that the 3-ft spacing 
should be measured along the ground from 
the outermost point on the dragline which 
a person could be expected to touch, not 
from the edge of the tub. The specific 
steps to be carried out are listed below. 

1. Connect one potential lead (P2) and 
one current lead (C2) of the resistivity 
meter directly to the machine or object 
to be tested, as shown in figure 3-1. 

2. Locate the fault current electrode 
(CI) at the point closest to the machine 
where a bare phase conductor is likely to 
come in contact with the earth. 

3. Locate the touch potential elec- 
trode (PI) 3 ft from the farthest pro- 
jection on the machine that a person 
can touch from the ground and directly 
between the machine and the fault elec- 
trode (position A). Record the instru- 
ment reading. 

4. Relocate the touch potential elec- 
trode (PI) 3 ft to either side of posi- 
tion A (positions B and C), and record 
the instrument readings there. If the 
readings at B and C are less than at A, 
then A is the location of maximum touch 
potential. 

5. If either B or C has a higher read- 
ing than A, then take an additional read- 
ing 1 m from the electrode with the high- 
er reading, on the side of the electrode 
opposite location A. 

6. If the reading at this fourth loca- 
tion is less than at location B or C 
(depending on which was higher than A), 
then B or C (again, depending on which 
was higher than A) is the location of the 
maximum touch potential. 

7. If the reading at this fourth loca- 
tion is greater still, then a fifth read- 
ing 1 m beyond the fourth is required. 
Continue this measurement pattern un- 
til a maximum reading is obtained. The 



location of the maximum reading is the 
location where the maximum touch poten- 
tial will occur. 

8. Measure the two-terminal resist- 
ance between the fault electrode (CI) and 
the machine. Leave C2 and P2 connected 
to the machine and connect PI to CI with 
a jumper. Record the reading. 

9. Calculate the fault current that 
will circulate between the fault and the 
machine. The fault current is equal to 
voltage of the phase conductor with re- 
spect to earth divided by the resistance 
between the fault and the machine. For 
example, if the (line-to-line) voltage is 
7,200 V, corresponding to line-to-neutral 
voltage of 4,160 V, and the measured re- 
sistance is 1,230 ohms, then the fault 
current is 4,160/1,230 = 3.38 A. 

10. The maximum touch potential that a 
person will experience is equal to the 
fault current multiplied by the maximum 
instrument reading obtained in steps 3 
through 7. 

Machine With a Square Footprint 

If the touch potential is to be mea- 
sured near an object with a square foot- 
print, the potential electrode should be 
placed in at least two different loca- 
tions. First, it should be positioned 
as above, closest to the current elec- 
trode but still 3 ft from the edge of the 
object. Next, the potential electrode 
PI should be placed 3 ft diagonally off 
the corner closest to the current elec- 
trode CI. If these first two potential- 
electrode positions are more than 3 ft 
apart, then the touch potential should be 
measured in a third position, halfway be- 
tween the first two locations. If this 
third position yields the highest instru- 
ment reading, then additional positions 
in the vicinity of this third location 
should be probed until the maximum is 
found. 

Fault Adjacent to Side of Square 

If the likely fault location is off the 
side of the square, the steps outlined 
below are appropriate. 



16 



1. Connect one potential lead (P2) and 
one current lead (C2) of the resistivity 
meter directly to the machine or object 
to be tested, as shown in figure 3-2. 

2. Locate the fault current electrode 
(CI) at the point closest to the machine 
where a bare phase conductor is likely to 
come in contact with the earth. 

3. Several locations for touch poten- 
tial measurements need to be investi- 
gated. Place the touch potential elec- 
trode (PI) at these locations: 

A. One meter from the machine di- 
rectly between the machine and the 
fault electrode (point A). 

B. One meter from the machine at 
the center of the side closest to the 
fault electrode (point B) . 

C. One meter from the machine at 
the two corners closest to the fault 
electrode (points C and D). 



Fault Adjacent to Corner of Square 

If the likely fault location is off a 
corner of the square, then the procedure 
shown next is appropriate. 

1. Connect one potential lead (P2) and 
one current lead (C2) of the resistivity 
meter directly to the machine or object 
to be tested, as shown in figure 3-3. 

2. Locate the fault current electrode 
(CI) at the point closest to the machine 
where a bare phase conductor is likely to 
come in contact with the earth. 

3. Locate the touch potential elec- 
trode (PI) at these points near the 
machine : 

A. One meter from the corner near- 
est the fault electrode (point A). 

B. One meter from the midpoints of 
the two sides nearest the fault elec- 
trode (points B and C) . 



Make a measurement at each of these loca- 
tions and find the location that gives 
the maximum reading. 



Make measurements at these locations and 
find the location that gives the maximum 
reading. 



4. Take additional measurements 1 m on 
either side of the maximum reading found 
in step 3. If these lateral measurements 
are less than the central measurement , 
then the central location is where the 
maximum touch potential will occur. 

5. If either of the lateral readings 
is higher than the central reading, then 
another measurement needs to be taken 1 m 
beyond the higher lateral reading. Con- 
tinue this measurement pattern until a 
maximum reading is found. 

6. Measure the two-terminal resistance 
between the fault electrode (CI) and the 
machine. Leave C2 and P2 connected to 
the machine and connect PI to CI. Record 
the reading. 

7. Calculate the fault current that 
will circulate between the fault and the 
machine. The fault current is equal to 
the voltage of the phase conductor with 
respect to earth divided by the resist- 
ance between the fault and the machine. 

8. The maximum touch potential that a 
person will experience is equal to the 
fault current multiplied by the maximum 
instrument reading obtained in steps 3 
through 5. 



4. Take additional measurements 1 m on 
either side of the maximum reading found 
in step 3. If these lateral measurements 
are less than the central measurement, 
then the central location is where the 
maximum touch potential will occur. 

5. If one of the lateral readings is 
higher than the central reading, then 
another measurement needs to be taken 1 m 
beyond the higher lateral reading. Con- 
tinue this measurement pattern until a 
maximum reading is found. 

6. Measure the two-terminal resistance 
between the fault electrode (CI) and the 
machine. Leave C2 and P2 connected to 
the machine and connect PI to CI. Record 
the reading. 

7. Calculate the fault current that 
will circulate between the fault and the 
machine. The fault current is equal to 
the voltage of the phase conductor with 
respect to earth divided by the resist- 
ance between the fault and the machine. 

8. The maximum touch potential that a 
person will experience is equal to the 
fault current multiplied by the maximum 
instrument reading obtained in steps 3 
through 5. 



17 



t >' 

lm lm 

J 



Machine 



+ 



Touch potential 
electrode 



c 

Overhead lines 
(bare conductor) 



Fault electrode 




CI 



FIGURE 3-2. - Measuring touch potential at a machine with a square footprint, fault nearest one side. 



Overhead lines 
(bare conductor) 




C2 P2PI CI 



Machine 



FIGURE 3-3. - Measuring touch potential at a machine with a square footprint, fault nearest one corner. 



18 



Machine With a Rectangular Footprint 

If the object has a rectangular foot- 
print, two more potential-electrode po- 
sitions should be added: 3 ft off the 
middle of a short side of the object, 
and 3 ft off the long side of the object. 
The short side and the long side chosen 
should be the closer ones to the current 
electrode. As above, measurements should 
be made at intermediate positions between 
these initial positions until the maximum 
instrument reading is obtained. 

Fault Adjacent to Long Side 
of Rectangle 

If the likely fault location is off the 
long side of the rectangle, follow the 
steps below. 

1. Connect one potential lead (P2) and 
one current lead (C2) of the resistivity 
meter directly to the machine or object 
to be tested, as shown in figure 3-4. 

2. Locate the fault current electrode 
(CI) at the point closest to the machine 
where a bare phase conductor is likely to 
come in contact with the earth. 

3. Several locations for touch poten- 
tial measurements need to be investi- 
gated. Place the touch potential (PI) at 
these locations: 

A. One meter from the machine di- 
rectly between the machine and the 
fault electrode (point A). 

B. One meter from the machine at 
the center of the side closest to the 
fault electrode (point B). 

C. One meter from the machine at 
the two corners closest to the fault 
electrode (points C and D). 

D. One meter from the machine at 
the center of the short side closest to 
the fault electrode (point E). 

Make measurements at these locations and 
find the location which gives the maximum 
reading. 

4. Take additional measurements 1 m on 
either side of the maximum reading found 
in step 3. If these lateral measurements 
are less than the central measurement, 
then the central location is where the 
maximum touch potential will occur. 



5. If one of the lateral readings is 
higher than the central reading, then 
another measurement needs to be taken 1 m 
beyond the higher lateral reading. Con- 
tinue this measurement pattern until a 
maximum reading is found. 

6. Measure the two-terminal resist- 
ance between electrode (CI) and the ma- 
chine. Leave C2 and P2 connected to the 
machine and connect PI to CI. Record the 
reading. 

7. Calculate the fault current that 
will circulate between the fault and the 
machine. The fault current is equal to 
the voltage of the phase conductor with 
respect to earth divided by the resist- 
ance between the fault and the machine. 

8. The maximum touch potential that a 
person will experience is equal to the 
fault current multiplied by the maximum 
instrument reading obtained in steps 3 
through 5. 

Fault Adjacent to Short Side 
of Rectangle 

If the likely fault location is off the 
short side of the rectangle, the steps 
outlined below are appropriate. 

1. Connect one potential lead (P2) and 
one current lead (C2) of the resistivity 
meter directly to the machine or object 
to be tested, as shown in figure 3-5. 

2. Locate the fault current electrode 
(CI) at the point closest to the machine 
where a bare phase conductor is likely to 
come in contact with the earth. 

3. Several locations for touch poten- 
tial measurements need to be investi- 
gated. Place the touch potential elec- 
trode (PI) at these locations: 

A. One meter from the machine di- 
rectly between the machine and the 
fault electrode (point A). 

B. One meter from the machine at 
the center of the side closest to fault 
electrode (point B). 

C. One meter from the machine at 
the two corners closest to the fault 
electrode (points C and D). 

Make measurements at these locations and 
find the location that gives the maximum 
reading. 



X 



m 



Overhead lines 
(bare conductor) 



Machine 



Touch potential electrode 



Fault 
electrode 




19 



FIGURE 3-4..- Measuring touch potential at a machine with a rectangular footprint, fault nearest a long side. 



Overhead lines 
(bare conductor) 



i 

Im 
±_ 



^( 



Machine 



I Touch potential electrode 



Fault 
electrode 




CI 



FIGURE 3-5. - Measuring touch potential at a machine with a rectangular footprint, fault nearest a short side. 



20 



4. Take additional measurements 1 m on 
either side of the maximum reading found 
in step 3. If these lateral measurements 
are less than the central measurement, 
then the central location is where the 
maximum touch potential will occur. 

5. If one of the lateral readings is 
higher than the central reading, then 
another measurement needs to be taken 1 m 
beyond the higher lateral reading. Con- 
tinue this measurement pattern until a 
maximum reading is found. 

6. Measure the two-terminal resistance 
between electrode (CI) and the machine. 
Leave C2 and P2 connected to the ma- 
chine and connect PI to CI. Record the 
reading. 

7. Calculate the fault current that 
will circulate between the fault and the 
machine. The fault current is equal to 
the voltage of the phase conductor with 
respect to earth divided by the resist- 
ance between the fault and the machine. 

8. The maximum touch potential that a 
person will experience is equal to the 
fault current multiplied by the maximum 
instrument reading obtained in steps 3 
through 5. 



4. Take additional measurements 1 m on 
either side of the maximum reading found 
in step 3. If these lateral measurements 
are less than the central measurement, 
then the central location is where the 
maximum touch potential will occur. 

5. If one of the lateral readings is 
higher than the central reading, then 
another measurement needs to be taken 1 m 
beyond the higher lateral reading. Con- 
tinue this measurement pattern until a 
maximum reading is found. 

6. Measure the two-terminal resistance 
between electrode (CI) and the machine. 
Leave C2 and P2 connected to the ma- 
chine and connect PI to CI. Record the 
reading. 

7. Calculate the fault current that 
will circulate between the fault and the 
machine. The fault current is equal to 
the voltage of the phase conductor with 
respect to earth divided by the resist- 
ance between the fault and the machine. 

8. The maximum touch potential that a 
person will experience is equal to the 
fault current multiplied by the maximum 
instrument reading obtained in steps 3 
through 5. 



Fault Adjacent to Corner of Rectangle 

If the likely fault location is off the 
corner of the rectangle, the steps out- 
lined below are appropriate. 

1. Connect one potential lead (P2) and 
one current lead (C2) of the resistivity 
meter directly to the machine or object 
to be tested as shown in figure 3-6. 

2. Locate the fault current electrode 
(CI) at the point closest to the machine 
where a bare phase conductor is likely to 
come in contact with earth. 

3. Locate the touch potential elec- 
trode (PI) at these points near the 
machine: 

A. One meter from the corner near- 
est the fault electrode (point A). 

B. One meter from the midpoints of 
the two sides nearest the fault elec- 
trode (points B and C). 

Make measurements at these locations and 
find the location which gives the maximum 
reading. 



Machine With an Irregular Footprint 

Irregularly shaped objects provide the 
greatest challenge. If the object has a 
shape which can be approximated by those 
discussed above, then the recommendations 
for that shape can be followed. Special 
attention should be given to narrow ex- 
tensions of the object footprint, such as 
outriggers on drills, dragline buckets, 
and conveyor belts. Measurements should 
be made off the ends of these extensions, 
beginning with those which are closest to 
the current electrode CI. It may be nec- 
essary to consider several current- 
electrode positions if the object is 
large and ground faults could occur at 
different points near the object. The 
potential electrode should be placed 3 ft 
from the end of any extension, regardless 
of whether the extension is in contact 
with the earth, as long as a person could 
stand on earth and touch the extension. 
The specific steps to be carried out are 
listed below. 



21 



Overhead lines 
(bare conductor) 




FIGURE 3-6. - Measuring touch potential at a machine with a rectangular footprint, fault nearest 
one corner. 



1. Connect one potential lead (P2) and 
one current lead (C2) of the resistivity 
meter directly to the machine or object 
to be tested. 

2. Locate the fault current electrode 
(CI) at the point closest to the machine 
where a bare phase conductor is likely to 
come in contact with the earth. 

3. Locate the touch potential elec- 
trode (PI) at these points near the 
machine: 

A. One meter from the machine di- 
rectly between the machine and the 
fault electrode (point A). 

B. One meter from the machine at 
all narrow projections or extensions. 

C. One meter from the machine at 
the center of the narrow sides of ma- 
chine nearest the fault electrode. 

Make measurements at these locations and 
find the location that gives the maximum 
reading. 

4. Take additional measurements 1 m on 
either side of the maximum reading found 



in step 3. If these lateral measurements 
are less than the central measurement, 
then the central location is where the 
maximum touch potential will occur. 

5. If one of the lateral readings is 
higher than the central reading, then 
another measurement needs to be taken 1 m 
beyond the higher lateral reading. Con- 
tinue this measurement pattern until a 
maximum reading is found. 

6. Measure the two-terminal resistance 
between electrode (CI) and the machine. 
Leave C2 and P2 connected 
chine and connect PI to CI 
reading. 

7. Calculate the fault 
will circulate between the fault and the 
machine. The fault current is equal to 
the voltage of the phase conductor with 
respect to earth divided by the resist- 
ance between the fault and the machine. 

8. The maximum touch potential that a 
person will experience is equal to the 
fault current multiplied by the maximum 
instrument reading obtained in steps 3 
through 5. 



to the ma- 
Record the 

current that 



22 



SPECIFIC RECOMMENDATIONS FOR 
SELECTED MINE TYPES 

Separate Ground Beds 

An additional test should be performed 
at mines that follow the practice (re- 
quired in coal mines) of having two sep- 
arate ground beds. The current electrode 
should be placed to simulate a ground 
fault as close as conceivable to the sta- 
tion ground, and the touch potentials 
measured at the safety ground. The po- 
tential electrode should be positioned as 
above, close to the current electrode, 
but 3 ft from the bed. This test will 
evaluate the effectiveness of ground-bed 
separation. Ground faults near the safe- 
ty ground bed should be probed as out- 
lined in the section, "General Guidelines 
for Electrode Placement," treating the 
bed as a "machine" with the same foot- 
print as the bed. 

Dredging Operations 

Although a dredge is surrounded by wa- 
ter, it is treated like any other machine 
in the touch-potential measurement (al- 
though the potential probe has to be lo- 
cated in the water). Special attention 
should be given to the side of the dredge 
where personnel get on or off, but high- 
est touch potentials will usually be 
encountered on the side of the dredge 
closest to the shore power feed. The 
current probe should be positioned for 
the closest ground fault to the dredge. 
This may be at a cable coupler close to 
the dredge, in addition to the usual 
overhead line locations. 

Open-Pit Mines 

For mines with ring feeds, care should 
be taken to ensure that the closest pos- 
sible ground fault be used when checking 
for touch potentials on pit equipment. 
An overhead line does not have to feed a 
particular machine to cause a shock haz- 
ard at the machine when it is downed. 



Underground Mines 

It is not practical to check touch po- 
tentials on underground machinery due to 
a surface ground fault. Measurements 
will have to be made at the ground bed on 
the surface. 

DETERMINATION OF FAULT-CURRENT VALUES 

The instrument readings obtained from 
the procedures previously outlined de- 
termine maximum values of mutual resist- 
ances; that is, touch potentials per 
ampere of fault current. Values of maxi- 
mum ground-fault current must be found 
to convert these instrument readings in- 
to touch potentials. There are three 
principal sources of these maximum cur- 
rents: (1) engineering estimates from 
line impedance and assumed fault imped- 
ance; (2) staged-fault tests; and (3) the 
experimental approach explained below. 
Of the three methods, the first is the 
most approximate, as it requires assuming 
a value for the ground-fault impedance. 
Because this impedance is a critical 
parameter in the calculation, large er- 
rors can be introduced. 

Staged-fault tests produce much more 
reliable numbers, but the potential dan- 
ger to personnel and the electrical 
stress on equipment make this an un- 
attractive alternative. Also, variations 
in earth resistivity and the length of 
phase conductor in contact with the earth 
produce a wide range of fault currents. 
However, for overhead lines above 15 kV, 
there is no alternative. 

The experimental approach is to experi- 
mentally measure the resistance seen by 
the faulted phase. Dividing the line- 
to-neutral voltage by this resistance 
produces an estimate of the ground-fault 
current. This approach works if the 
ground-fault resistance is much larger 
than the source impedance (usually true) 
and the line voltage is low enough so 
that ionization of the earth is not ap- 
preciable (usually true for 15 kV or 
less). Merits of this method are that 



23 



situations, such as phase conductors 
falling on ungrounded metal objects, can 
be explicitly considered. 

Figure 3-7 illustrates the procedure. 
A current is circulated between the sys- 
tem neutral and an electrode correspond- 
ing to the downed phase conductor. This 
electrode may be a length of bare con- 
ductor on the ground, representing the 
phase conductor, or may be a metal object 
onto which a phase conductor could fall. 
(The chosen object should not be con- 
nected to the grounding system, as this 
would not produce substantial fault cur- 
rent through-the-earth; the fault would 
be line-to-neutral.) The ground fault 
should be located at the worst-case posi- 
tion of the current electrode, determined 
above. Note that the grounding resistor, 
if present, would be correctly included 
in the ground-fault resistance. 

The resistance measured above should be 
divided into the highest line-to-neutral 
voltage expected on a continuous basis. 
Because the measured resistance is a low 
estimate of the fault impedance, the 
fault current calculated will be a con- 
servative (high) estimate of the maximum 
fault current. This estimated current 
may be unreasonably high if the measured 
resistance is very low, owing to the 
neglected impedance of the transmission 
and distribution system. 

INTERPRETATION OF RESULTS 

The maximum instrument reading obtained 
previously, multiplied by the maximum 
ground-fault current, yields the worst- 
case touch potential. If the maximum 



fault current was found from the experi- 
mental procedure of the last section, the 
results should be reviewed to verify that 
the ground-fault resistance was measured 
at the same location as the current elec- 
trode was placed for the original test. 

Touch potentials of 100 V or more are 
unacceptable. If the tests reveal values 
close to, but less than, 100 V, the 
test procedure should be reviewed and 
additional measurements should be taken 
to ensure that the worst-case values have 
indeed been found. Values approaching 
100 V can be lethal in a wet or otherwise 
low-resistivity area. 

MITIGATION OF TOUCH-POTENTIAL HAZARDS 

Reduction of touch potentials, or re- 
ducing the hazards due to these poten- 
tials, cannot be fully treated here. 
Listed below are some methods of attack- 
ing the problem. In general, the hazard 
is reduced by decreasing the ground-fault 
current, by decreasing the mutual resist- 
ance (voltage per ampere of fault cur- 
rent) , or else by keeping personnel away 
from an area of high potentials. 

Touch potentials can be reduced by in- 
creasing the separation of overhead lines 
and equipment (or ground beds) from pos- 
sible ground-fault locations. Moving 
either the overhead line or the equip- 
ment will reduce the hazard. If the high 
touch potential is confined to a small 
area, such as the dragline bucket near 
an overhead line, keeping personnel away 
from the area may be effective. Adding 
additional rods to narrow sides of ground 
beds will reduce touch potentials near 




/VAVWAW/AW/AW/AW/AW/AW/A 



Safety 
ground ' 
bed 



Fault 
electrode v 



V/AW/AW/A\ 



FIGURE 3-7. - Determination of expected fault current. 



24 



these sides. Reducing ground-fault cur- 
rent, by moving metal objects out of the 
right-of-way of distribution lines (to 
increase the ground-fault impedance) , by 
increasing grounding-resistor value (to 
increase the resistance in series with 
the fault) , or by placing gravel in low- 
resistivity areas under lines (to in- 
crease ground-fault impedance) will also 
decrease touch potentials. 

If these techniques do not reduce the 
hazards sufficiently, redesign of the 
grounding system may be necessary. Sepa- 
ration of grounds may be advantageous; in 



some cases, the opposite procedure of 
combining separate ground beds (if per- 
mitted) may provide relief. 

CONCLUSION 

In this paper a new method to assess 
earth grounding safety has been fully 
documented in cookbook format. Since a 
commercially available meter is employed, 
the procedure can be readily adopted by 
the Mine Safety and Health Administration 
(MSHA) and the mining industry. 



REFERENCES 



1. American Standards Association, 
American Mining Congress, and U.S. Bu- 
reau of Mines. American Standard Safety 
Rules for Installing and Using Electri- 
cal Equipment in and About Coal Mines. 
BuMines IC 8227, 1964, 27 pp. 

2. Lordi, A. C. How To Safely Ground 
Mine Power Systems. Coal Age, v. 68, 
Sept. 1963, pp. 110-117. 

3. King, R. L., H. W. Hill, Jr., R. R. 
Bafana, and W. L. Cooley. Guide for the 
Construction of Driven-Rod Ground Beds. 
BuMines IC 8767, 1978, 26 pp. 

4. Cooley, W. L. , and R. L. King. 
Guide to Substation Grounding and Bonding 
for Mine Power Systems. BuMines IC 8835, 
1980, 27 pp. 

5. Mitchell, J. B. , H. W. Hill, Jr., 
and W. L. Cooley. Composite Material 



Ground Beds for Difficult Areas. Paper 
in Proceedings of the Fifth WVU Con- 
ference on Coal Mine Electrotechnology , 
July 30-31, August 1, 1980, ed. by N. S. 
Smith. BuMines OFR 82-81, 1980, pp. 10-1 
to 10-18. 

6. Mitchell, J. B. Composite Material 
Ground Beds for Low-Conductivity Soils. 
M.S. Thesis, WV Univ., 1981, 103. 

7. Sunde, E. D. Earth Conduction Ef- 
fects in Transmission Systems. Dover 
Publ. Inc., 1967, 370 pp. 

8. James G. Biddle Co. Getting Down 
to Earth. Plymouth Meeting, PA, 1970, 
48 pp. 

9. Hill, H. W. , Jr. Private communi- 
cation, 1985; available upon request from 
M. R. Yenchek, BuMines, Pittsburgh, PA. 



&U.S. CPO: 1985-505-019/20,109 



INT.- BU. OF MINES, PGH..PA. 28 111 



IK. 



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Bureau of Mines— Prod, and Distr. 
Cochrans Mill Road 
P.O. Box 18070 
Pittsburgh. Pa. 15236 



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